OK, Bipolar disorder is thought to be caused by neurochemical problems in the dopamine, serotonin, and norepinephrine systems. The treatment is typically medications, with or without psychotherapy. Mood states might be triggered by stress, but once a person is in the state it can be tenacious. It can be particularly hard to distinguish mixed states from BPD, because the mood will be so unstable, but it is unstable biochemically as opposed to reactively.

The difference between Bipolar I and Bipolar II - Bipolar I is when a person has at least one full blown manic episode or mixed state (symptoms of depression and mania at the same time), with or without a history of depression. Bipolar II is when the person has at least one episode of major depression and at least one episode of hypomania, but no history of mania. Bipolar II can not be thought of as simply a less extreme for of the illness because the depressions can be just as severe, and Bipolar II typically spends more times in depression than Bipolar I. Bipolar tends to get worse with age if it is untreated, while BPD tends to get better with age/burn itself out. In Bipolar, psychotic symptoms are found only during times of mood extremes, but may be severe and include hallucinations or delusions - although not all people with Bipolar get psychotic (For example, I have had brief psychosis, but it is very rare for me).

Ok - criteria

Major depressive episode - 2 weeks+ of depressed mood or loss of interest, plus combo of
disturbed sleep
appetite changes
poor concentration
suicidal ideation
feelings of guilt or worthless restlessness
psychomotor agitation or slowing
poor energy

Criteria for mania (hypomania is a less severe version of the same symptoms):

Elevated or irritable mood for at least 4 days or any length requiring hospitalization, plus combos of:

Reduced need for sleep
Racing thoughts
Pressured speech/more talkative
Flight of ideas
Increased goal directed behavior
Increased high risk, pleasurable behavior (spending, sex, substance use, etc)
Increased self-esteem
Distractibility

In bipolar the symptoms will typically be absent during times when the mood is stable.

Anyways, there is a description of the 2 disorders. It can be hard to tell apart in real life because the person may be suicidal, impulsive, into high risk activities, depressed, etc in both. The main reason for differentiating between the 2 is because the treatments are different.

I hope this explanation helps.